Snowthrowers are known having a pair of drive wheels for propelling the snowthrower over the ground. One drive wheel is carried on each side of the snowthrower. Typically, the rotational speed of the drive wheels can be adjusted by the operator using a speed control lever to adjust the ground speed of the snowthrower. In addition, a separate engagement control lever is provided for the operator to selectively engage and disengage the drive wheels. However, once the operator has selected a particular speed for the wheels using the speed control lever and has engaged the wheels using the engagement control lever, the wheels continuously rotate at the same speed.
Many such prior art snowthrowers cannot be steered by the operator other than by pushing or pulling on the handles of the snowthrower while the drive wheels are rotating. For example, to effect a turn to the left, the operator would typically push on the handles to turn the front of the snowthrower to the left. This can be physically demanding for some operators particularly in the snowy conditions in which snowthrowers are used. In addition, the steering provided by such a method typically results in jerky turning motions of the snowthrower to one side or the other. It is thus somewhat difficult to precisely steer the typical snowthrower having a pair of continuously rotating drive wheels.
Some prior art snowthrowers incorporate a steering system that permits the snowthrower to be more easily steered. For example, in some snowthrowers, the drive wheels are driven through a differential. The differential automatically reduces the speed of the drive wheel on the inside of a turn and automatically increases the speed of the drive wheel on the outside of a turn. Thus, while the operator still steers the snowthrower by pushing on the handles, the differential facilitates the turn and allows more precise directional control.
Unfortunately, differentials are relatively expensive and unduly increase the cost of the snowthrower. In addition, when one drive wheel driven by a differential spins or loses traction, the differential automatically transfers torque to the spinning drive wheel from the non-spinning drive wheel. Snowthrowers often operate on icy or snowy surfaces where one drive wheel might hit a patch of ice or snow and lose traction compared to the other drive wheel which retains traction. In this case, the snowthrower loses all traction since the differential transfers all the torque to the spinning drive wheel. As a result, a differential equipped snowthrower is not an effective way in a practical sense to steer a snowthrower.
U.S. Pat. No. 5,018,592 discloses another way to steer a snowthrower. In the 592 patent, each drive wheel is independently driven by a planetary gear transmission. A steering lever provided on each handle is coupled to the planetary gear transmission on the same side of the snowthrower. When the steering lever on one handle is manipulated by the operator, a pawl is removed from engagement with a ring gear to allow the ring gear to rotate and thereby place the planetary gear transmission on that side of the snowthrower into an inoperative, non-driving condition. This effectively unclutches the drive wheel on the inside of the turn to effect a skid type turn, i.e. the drive wheel on the inside of the turn is not powered while the drive wheel on the outside of the turn remains powered.
While the steering system shown in the 592 patent is effective for steering a snowthrower, a planetary gear transmission is relatively complex. In addition to the ring gear, the planetary gear transmission requires a sun gear and a plurality of planet gears between the sun gear and the ring gear. Thus, a durable, simple and less expensive system for steering a snowthrower is desirable.